Material properties of the cell walls in nanofibrillar cellulose foams from finite element modelling of tomography scans

The mechanical properties of the nanofibrillar cellulose foam depend on the microstructure of the foam and on the constituent solid properties. The latter are hard to extract experimentally due to difficulties in performing the experiments on the micro-scale. The aim of this work is to provide methodology for doing it indirectly using extracted geometry of the microstructure. X-ray computed tomography scans are used to reconstruct the microstructure of a nanofibrillar cellulose foam sample. By varying the levels of thresholding, structure of differing porosities of the same foam structure are obtained and their macroscopic properties of the uni-axial compression are computed by finite element simulations. A power law relation, equivalent to classical foam scaling laws, are fit to the data obtained from simulation at different relative densities for the same structure. The relation thus obtained, is used to determine the cell wall material properties, viz. elastic modulus and yield strength, by extrapolating it to the experimental porosity and using the measured response at this porosity. The simulations also provide qualitative insights into the nature of irreversible deformations, not only corroborating the experimental results, but also providing possible explanation to the mechanisms responsible for crushable behaviour of the nanofibrillar cellulose foams in compression.